Xen ARM with Virtualization Extensions
The ARM v7-A architecture includes the optional virtualization extensions that allow a hypervisor to manage fully hardware virtualized guests. These extensions will be available in some ARM Cortex A15 and Cortex A7 processors.
This port is a work-in-progress. Xen and Dom0 boot on Versatile Express Cortex A15 hardware and unprivileged guests can be created and destroyed using xl.
The guest (including dom0) visible ABI exposed by the hypervisor is not yet set in stone and there may be changes as work progresses. See requirements (below) for specific details of matching kernel and hypervisor versions.
Currently Xen runs on:
- The Real-time System Model's provided by ARM. See Xen ARMv7 with Virtualization Extensions/FastModels for more information on obtaining and building the necessary model.
- A Cortex-A15 Processor running on the Versatile Express.
- Here is some step to start Xen on the Arndale Development board
- ARM Fast Model
- Either the v7-A ARM Envelope Model (AEM) or the Cortex A15/A7 models (single CPU only).
- Device Tree
- A device tree in the flat device tree format (.dtb). The device tree source for unprivileged Xen ARM guests is available upstream in the Linux tree >= v3.7: xenvm-4.2.dts. The interface is also documented in the Linux tree: xen.txt. Regarding Xen and Dom0, the regular DTS for the underlying platform can be used, however it requires the additional hypervisor node described in the Linux Documentation and the command line arguments under the chosen node, see for example the Xen DTS for the Versatile Express Cortex A15 machine we use for development vexpress-v2p-ca15-tc1.dts.
- All current work is now merged into the current development branch  or . It is recommended to use the latest xen-unstable commit.
- Linux kernel for dom0
- The patches necessary to boot Linux as dom0 under Xen were merged upstream in v3.7. However in order to actually start guests a few additional patches are required. These patches can be found in the arm-privcmd-for-3.8 branch of git://xenbits.xen.org/people/ianc/linux.git and they went upstream in Linux v3.8-rc1. The latest Linus' tree has everything needed to run on Xen on ARM as dom0 and domU.
- dom0 userspace
- The developers are using the armhf port of Debian Wheezy (which is currently the testing release).
- domU kernel
- The patches necessary to boot Linux as a guest under Xen were merged upstream in v3.7.
The ABI for Xen on ARM is currently not set in stone. The branches and trees referenced above are self consistent.
Xen needs the device trees to be in the flat device tree format (the device tree blob or DTB). This is built by the device tree compiler (dtc) from the device tree source files (.dts and .dtsi).
Version 1.3 or later of dtc is required.
Build the .dtb files with make.
When running on the model there is no bootloader to provide the DTB. Instead, it is linked with the xen image.
The DTB to use is specified by the CONFIG_DTB_FILE config variable which should be set to the absolute path of the DTB. This variable may be set in .config or on the make command line.
The Xen command line may be set with the bootargs parameter of the chosen node of the device tree.
Enable at least ARCH_VEXPRESS, ARCH_VEXPRESS_DT. Disable SPARSE_IRQ (it doesn't play nicely with device tree support). If ARM_APPENDED_DTB is enabled then any appended DTB will be used instead of one supplied by Xen and the kernel will crash unless the memory in the DTB matches that location/size supplied by Xen.
A working configuration is available here.
The Linux command line should be set with the xen,dom0-bootargs parameter of the chosen node of the device tree supplied to Xen. If this parameter is missing then Linux will not get a command line and the wrong console will be used (there's a bug where the kernel doesn't use the default, built-in command line if the /chosen node is missing.
The dom0 kernel is loaded by Xen from the beginning of the flash.
To boot the kernel natively use an updated boot-wrapper from git://xenbits.xen.org/people/dvrabel/boot-wrapper.git.
Build the device tree blobs provided with the kernel with make dtbs. A suitable DTB file for the envelope model is vexpress-v2p-aem-v7a.dtb.
For a native boot the kernel should have ARM_APPENDED_DTB and ARM_ATAG_DTB_COMPAT enabled. The kernel zImage and the DTB should be appended. e.g., cat zImage vexpress-v2p-aem-v7a.dtb > image.bin.
Building Xen on ARM
Cross compiling the hypervisor is relatively simple. You can find a cross compiler in some Linux distros or you can download the arm-unknown-linux-gnueabi compiler from kernel.org.
Once you have a suitable cross compiler you can compile Xen with:
$ make dist-xen XEN_TARGET_ARCH=arm32 CROSS_COMPILE=arm-unknown-linux-gnueabi-
This assumes that the command prefix for you cross compiler is arm-unknown-linux-gnueabi- and that arm-unknown-linux-gnueabi-gcc and friends are in your $PATH.
Unfortunately cross-compiling the tools is not currently supported
In order to build the tools a native build environment is required. The developers mainly use the armhf port of Debian, which is present in Wheezy (the current testing distribution) running on an IMX53 based development board, although any ARMv7 development board would do. Note that the build hardware does not need to support the virtualisation extensions, since you don't have to run Xen on the same system as where you build it.
It may also be possible to build using a cross-architecture chroot running on an X86 system as described in this blog post, although this hasn't been tried yet.
Known Problems and Workarounds
- SMP support is under development in Xen ARM as we speak, we recommend exporting only 1 cpu in the DTS for the moment.
- Linux fails to change the MMC voltage during boot on Versatile Express, a patch is needed to fix the bug.
- The address of the Dom0 kernel in Flash at the moment is not configurable, it is defined as KERNEL_FLASH_ADDRESS in Xen. On a Versatile Express machine this address can be determined by executing flash list images from the boot manager. You might have to manually change it.
Use the Xen ARMv7 Dev FAQ page to cover commonly asked questions.